Closed loop ceps-press control systems

ABSTRACT

An integrated computerized system for use in color printing having at least one digital representation of a color characteristic of at least one page to be printed and a digital storage memory for storing the digital representation. An imposition apparatus is connected to the storage memory to receive the digital representation and arrange the digital representation in accordance with a desired plate layout, thereby to define a plate image. A press set-up device extracts from the plate image the color characteristic and provides ink flow set-up data to a printing press in accordance with the extracted color characteristic.

This is a continuation of application Ser. No. 10/329,264 filed Dec. 23,2002, which is a continuation of application Ser. No. 09/736,610 filedDec. 14, 2000, now U.S. Pat. No. 6,545,772, which is a division ofapplication Ser. No. 09/224,689 filed Jan. 4, 1999, now U.S. Pat. No.6,181,439, which is a division of application Ser. No. 07/650,249 filedFeb. 4, 1991, now U.S. Pat. No. 5,875,288, claims the benefit thereofand incorporates the same by reference.

FIELD OF THE INVENTION

The present invention relates to printing and graphic arts generally andmore particularly to apparatus for page composition and printing andapparatus for printing control.

BACKGROUND OF THE INVENTION

The production of a book or magazine involves a large number ofprocesses. The processes may be grouped into five general categories:prepress, plate preparation, press set up or “make ready”; press controloperations; and postpress operations.

In prepress, the principal processes are graphics, image and textediting and composition. In recent years, this area has undergonerevolutionary changes through application of computer technology. Stateof the art systems for graphics and image editing and composition, aswell as the specific functions of scanning, image processing andplotting include systems manufactured and sold by Scitex CorporationLtd., Herzlia, Israel, such as, respectively, the systems sold under thetrademarks Smart Scanner, Assembler and Raystar.

Plate preparation involves a technique known as imposition or signatureassembly, which refers to the arrangement of pages on a film used toproduce a printing plate. The present state of the art in signatureassembly is the use of large and expensive machinery requiring highlyskilled operators. Presently available signature assembly machineryinvolves the following steps: exposure of each of a plurality ofseparations (such as C, M, Y and K) for each of a plurality of pag s tobe arranged on a single plate; and subsequently, for each separation,such as C, feeding the exposed plurality of pages to an expose andrepeat device such as the Misomex Master S Series, commerciallyavailable from Misomex North America Inc., Rosemont, Ill. The expose andrepeat device is operative to arrange the plurality of exposed Cseparations, corresponding to the plurality of pages to be arranged on asingle plate, at precisely determined positions on a film and toreproduce the C separations at the precisely determined positions by aphotographic process.

The C printing plate is produced by contact exposure of the film. Thesignature is proofed prior to exposure of the plate. The plate, onceexposed, is often found to be imperfect and therefore generally requiresquality inspection and defect repair processes. The plate preparationprocess is repeated in order to produce M, Y and K plates.

Press set-up or “make-ready” takes place after preparation of theprinting plate, and involves scanning of each printing plate to obtaininformation necessary for press set-up. This information includes thevariation of the dot percentage over each plate, which determines therequired ink flow for that plate.

Press control involves adjustments to the printing press which arenormally made by a pressman examining the printed output from the pressor by automatic press control systems such as the SPM 700, commerciallyavailable from Gretag Data and Image Systems, CH-Regensdorf,Switzerland. Automatic press control systems are operative to scan theprinted output from the press compare it to a reference, such as asignature proof, and make suitable adjustments to the printing press.Post press processes include the folding, cutting and binding of thepress output.

A great number of patents exist in the area of prepress. These includeco-assigned U.S. Pat. No. 4,456,924 which describes method and apparatusfor screen generation, co-assigned U.S. Pat. No. 4,853,709 whichdescribes an internal drum laser plotter, and co-assigned U.Sapplication Ser. No. 044,428 which describes a color digital scanner.

Various processes in the production of printed matter are discussed inthe following publications, the disclosures of which are incorporatedherein by reference:

Alfred Furler, Folding in Practice, in collaboration with STAHL GMBH &Co., 3rd Edition, Germany, 1983;

William Schreiber, Fundamentals of Electronic Imaging Systems,Springer-Verlag, New York, 1986;

R. K. Molla, Electronic Color Separation, RK Publications, 1988; and

Michael H. Brill (Ed.), Perceiving, Measuring and Using Color, SPIEProceedings, Vol. 1250, Billingham, Wash., USA, February 1990;

Robert A. Ulicheney, Digital Halftoning and Physical ReconstructionFunction, Ph.D. Thesis, Massachusetts Institute of Technology, MA, USA,June 1986; and

Raymond Blair and Charles Shapiro (Eds.), The Lithographer's Manual,GATF, USA, 1980.

A state of the art printing press is described in U.S. Pat. No.4,936,211 to Pensavacchia et al. Pensavacchia et al mention printingapparatus which is intended to achieve complete computer control overthe entire printing process, including plate generation, ink regulationand the start-up, print, hold, shut-down and clean-up stages of theactual printing operation. Pensavacchia et al also refer to a pressincluding a workstation which allows an operator to input arepresentation of an original picture to be printed. The workstation mayinclude a CRT display and internal memory for storing image data so thatthe impression to be printed may be previewed before printing and akeyboard via which the operator may key in instructions regarding theparticular press run such as the number of copies to be printed or thenumber of colors in the printed copies. The workstation also is intendedto allow complete control over the operating modes of the pressincluding printing plate imaging, press startup procedure, ink flowregulation, dampening, print, pause and shutdown and clean-up sequences.

Computer technology has also been proposed for use in facilitatingcertain stages of the imposition process as described in EuropeanPublished Patent Application 0309196, published Mar. 29, 1989, EuropeanPublished Patent Application 0348908, published Jan. 3, 1990 and in U.K.Patent 2128843, published on May 2, 1984. U.S. Pat. No. 4,150,991describes one type of optical imposition machine employingtransparencies.

SUMMARY OF THE INVENTION

The present invention seeks to provide an integrated computerized systemfor carrying out multiple stages of a printing process. In accordancewith a preferred embodiment of the invention, the integratedcomputerized system is capable of carrying out functions in all stagesof the printing process from pre-press to press control.

There is thus provided in accordance with a preferred embodiment of thepresent invention an integrated computerized system for use in printingincluding apparatus for providing at least one digital representation ofat least one page, digital storage apparatus for storing the at leastone digital representation of the at least one page, impositionapparatus for receiving at least one digital representation of at leastone page and arranging the at least one digital representation of the atleast one pages in accordance with a desired plate layout, thereby todefine a plate image, and press set-up apparatus for extracting from theat least one digital representation of at least one page and providingto a printing press, press set-up data.

There is also provided in accordance with a preferred embodiment of thepresent invention an integrated computerized system for use in printingand including apparatus for providing at least one digitalrepresentation of at least one page, digital storage apparatus forstoring the at least one digital representation of at least one page,imposition apparatus for receiving the at least one digitalrepresentation of at least one page and arranging the at least onedigital representation of at least one page in accordance with a desiredplate layout, thereby to define a plate image, and press controlapparatus for automatically monitoring the printed output of theprinting press and automatically providing adjustments to printing pressregistration and ink flow controls in accordance therewith.

Further in accordance with a preferred embodiment of the presentinvention, the system also includes press control apparatus forautomatically monitoring the printed sheet provided by the printingpress and for providing adjustments to printing press registration andink flow controls in accordance therewith.

Still further in accordance with a preferred embodiment of the presentinvention, the at least one digital representation of at least one pageincludes a plurality of digital representations of a correspondingplurality of pages.

Additionally in accordance with a preferred embodiment of the presentinvention, the at least one digital representation of at least one pageincludes at least one digital representation of a signature marking.

Still further in accordance with a preferred embodiment of the presentinvention, the at least one digital representation of a signaturemarking includes a digital representation of a control strip.

Additionally in accordance with a preferred embodiment of the presentinvention, the apparatus for providing includes editing apparatus forreceiving at least one of text and graphics and providing an edited pagelayout.

Further in accordance with a preferred embodiment of the presentinvention, the editing apparatus includes a scanner and/or a workstationreceiving an input from a scanner and operator inputs and beingoperative to provide a digitally storable page layout.

Still further in accordance with a preferred embodiment of the presentinvention, the press set-up apparatus includes data base apparatus fortaking into account characteristics of at least one of inks andsubstrates to be used by the press.

Additionally in accordance with a preferred embodiment of the presentinvention, the press set-up apparatus also includes apparatus for takinginto account characteristics of the press.

Further in accordance with a preferred embodiment of the presentinvention, the press control apparatus includes apparatus for extractinginformation from the at least one digital representation of at least onepage, apparatus for comparing a press output to the extractedinformation, and apparatus for adjusting the press in accordance with anoutput indication provided by the apparatus for comparing.

There is also provided in accordance with yet a further preferredembodiment of the present invention an integrated computerized systemfor use in process and monochrome color printing and including apparatusfor providing at least one digital representation of at least one page,digital storage apparatus for storing at least one digitalrepresentation of at least one page, imposition apparatus for receivingat least one of the at least one digital representations of at least onepage and arranging the at least one digital representation of at leastone page in accordance with a desired plate layout, and automatic screencharacteristic selection apparatus operative to select at least onescreen characteristic for each of a plurality of regions defined withinat least one separation of the at least one digital representation of atleast one page.

Further in accordance with a preferred embodiment of the presentinvention, the system includes automatic screen characteristic selectionapparatus operative to locally select at least screen angles for each ofat least one region in the plate layout based on the color content ofthe at least one region.

Still further in accordance with a preferred embodiment of the presentinvention, the imposition apparatus includes apparatus for providing adigital representation of the plate image and plate quality controlapparatus for comparing the digital representation of the plate image toinformation extracted from the printed sheet provided by the printingpress.

According to still a further preferred embodiment of the presentinvention, there is provided imposition apparatus including apparatusfor receiving at least one digital representation of at least one page,apparatus for receiving a desired plate layout, and apparatus forproviding a digital representation of a plate image corresponding to theat least one digital representation of the at least one page arranged inaccordance with the desired plate layout.

Further in accordance with a preferred embodiment of the presentinvention, the apparatus for receiving includes apparatus for receivingonly a portion of at least one digital representation of at least onepage and the apparatus for providing a digital representation includesapparatus for providing a digital representation of a portion of theplate image in accordance with the desired arrangement of the at leastone page portion on the portion of the plate.

Still further in accordance with a preferred embodiment of the presentinvention, the imposition apparatus includes apparatus for receiving atleast one digital representation of at least one page, apparatus forreceiving information pertaining to a desired plate layout, andapparatus for providing a digital representation of a plate imagecorresponding to the at least one digital representation of the at leastone page arranged in accordance with the desired plate layout.

Additionally in accordance with a preferred embodiment of the presentinvention, the information includes at least information regardingfolding characteristics and/or information regarding cuttingcharacteristics, and/or information regarding binding characteristics.

Still further in accordance with a preferred embodiment of the presentinvention, the at least one digital representation of at least one pageincludes a plurality of digital representations of a correspondingplurality of pages and the imposition apparatus also includes apparatusfor modifying the format of at least one of the plurality of digitalrepresentations of pages.

Additionally in accordance with a preferred embodiment of the presentinvention, the apparatus for receiving information includes apparatusfor providing a proof of at least the plate image and/or the platelayout.

Further in accordance with a preferred embodiment of the presentinvention, the proof of the plate image includes an analogrepresentation of the content of the at least one page derived from thedigital representation of the content of the at least one page.

Still further in accordance with a preferred embodiment of the presentinvention, the plate image includes a digital representation of at leastone signature marking.

There is also provided in accordance with a preferred embodiment of thepresent invention color separation generating apparatus includingapparatus for generating a color separation, the color separationdefining a plurality of regions characterized in that at least onescreen characteristic within a first individual one of the plurality ofregions differs from the at least one screen characteristic within asecond individual one of the plurality of regions.

There is further provided in accordance with a preferred embodiment ofthe present invention automatic screen characteristic selectionapparatus including apparatus for inspecting at least a portion of arepresentation of a color image, and apparatus for receiving an outputindication from the apparatus for inspecting and using the outputindication to select at least one screen characteristic for at least aportion of at least one separation of the color image.

Further in accordance with a preferred embodiment of the presentinvention, the apparatus for inspecting is operative to inspect aportion of a color image and the apparatus for receiving is operative toselect at least one screen characteristic for the corresponding portionof at least one separation of the color image.

Still further in accordance with a preferred embodiment of the presentinvention, the apparatus for receiving includes apparatus for selecting,for at least one separation of the color image, at least one screencharacteristic for each of a plurality of regions of the color imagesuch that the at least one screen characteristic within a firstindividual one of the plurality of regions differs from the at least onescreen characteristic within a second individual one of the plurality ofregions.

Additionally in accordance with a preferred embodiment of the presentinvention, the representation of the color image includes a digitalrepresentation of the color image.

Further in accordance with a preferred embodiment of the presentinvention, the at least one screen characteristic includes at least ascreen angle, and/or a screen dot shape.

Still further in accordance with a preferred embodiment of the presentinvention, the apparatus for inspecting includes apparatus forinspecting the color content of at least a portion of a representationof a color image and the apparatus for receiving includes apparatus forselecting at least one screen characteristic for at least one separationof the color image according to the color content of the portion of therepresentation of the color image.

There is further provided in accordance with still a further preferredembodiment of the present invention signature characteristic unificationapparatus including apparatus for inspecting at least a portion of arepresentation of a signature, and apparatus for modifying at least oneimage characteristic within at least one region of the representation ofthe signature, thereby to unify the at least one image characteristicover at least a portion of the signature.

There is also provided in accordance with a preferred embodiment of thepresent invention, signature characteristic unification apparatus, thesignature including a representation of a plurality of pages, theapparatus including apparatus for modifying at least one representationcharacteristic within at least one region of at least one first pagefrom among the plurality of pages, thereby to unify the at least oneimage characteristic relative to at least one region of at least onesecond page from among the plurality of pages.

Further in accordance with a preferred embodiment of the presentinvention, the at least one representation characteristic includes atleast one color characteristic.

Still further in accordance with a preferred embodiment of the presentinvention, the at least one representation characteristic includes atleast one spatial characteristic.

Additionally in accordance with a preferred embodiment of the presentinvention, the at least one spatial characteristic includes a resolutionof a color image.

There is also provided in accordance with a preferred embodiment of thepresent invention an image reproduction method including the steps ofproviding a representation of an image, the representation including adigital indication of at least one location whose appearance is to bemaintained, reproducing the image so as to automatically maintain theappearance of the location.

Further in accordance with a preferred embodiment of the presentinvention, the representation of the image includes a digitalrepresentation of the image and the step of reproducing includes thestep of providing the digital representation of the location toapparatus for reproducing the image.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be understood and appreciated more fully fromthe following detailed description, taken in conjunction with thedrawings in which:

FIG. 1 is a simplified block diagram illustration of an integratedcomputerized system for use in printing constructed and operative inaccordance with a preferred embodiment of the present invention;

FIG. 2 is a flowchart of a preferred embodiment of a “first pass”algorithm useful in implementing digital signature assembly generator 16of FIG. 1 and comprising a procedure for file reading and rearrangingand for screen angle processing;

FIG. 3 is a flowchart of a non-rotating block providing procedure usefulin conjunction with the algorithm of FIG. 2;

FIG. 4 is a flowchart of a rotating block providing procedure useful inconjunction with the algorithm of FIG. 2;

FIG. 5 is a flowchart of a preferred embodiment of a “second pass”algorithm useful in implementing digital signature assembly generator 16of FIG. 1 and comprising a procedure for providing digital signatureinformation to screen generator 20 of FIG. 1;

FIG. 6 is a flowchart of a preferred embodiment of the screen subroutineof step 66 of the flowchart of FIGS. 3; and

FIG. 7 is a preferred embodiment of the subroutine of steps 124 and 130of FIG. 5.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

The following terms, employed herein, are intended to have the meaningsspecified hereinbelow:

Color Image

This term is intended to include image comprising gradations of a singletone, such as black and white images.

Analog Representation of a Color Image

Any representation of a color image which resembles the original colorimage. The representation may appear upon a printed page, a proof or anyother suitable substrate.

Digital Representation of a Color Image

Any representation of a color image which is expressed in discretesymbols, such as a computer file.

Color Characteristics of a Color Image

The characteristics of the color image defined by individual elements ofa representation of a color image which directly represent a color or acomponent of a color.

Spatial Characteristics of a Color Image

Characteristics defining the arrangement of and the relationship betweenelements of a representation of a color image, such as pixels, whichcharacteristics do not directly represent a color or a component of acolor. Spatial characteristics include but are not limited to resolutionand format characteristics such as pixel by pixel encoding.

Digital Output Device

Apparatus which inputs a digital representation of a color image andconverts it into an analog representation thereof, such as but notlimited to a plotter or proofer. The analog representation may beprovided on any suitable substrate such as a hard copy proof, film orplate.

Reference is now made to FIG. 1, which illustrates an integratedcomputerized system for use in printing constructed and operative inaccordance with a preferred embodiment of the present invention andincluding apparatus 10 for providing a plurality of single page digitalrepresentations 12. Apparatus 10 typically comprises at least oneconventional computerized page layout and assembly system, such as theAssembler workstation commercially available from Scitex Ltd.Alternatively, non-Scitex products such as Illustrator and Photoshop,both commercially available from Adobe, and Freehand, commerciallyavailable from Aldus may be employed in conjunction with an interpreterdevice such as PS-Bridge, commercially available from Scitex.

One or more such computerized page layout systems may be provided orlinked to the system of the present invention by any suitable datacommunication technique or means, and may be remotely located from therest of the system, as desired. Alternatively, the single-page digitalrepresentations may arrive from a large storage device such as a disk.

Color and spatial basis unification apparatus 14 receives the pluralityof single-page digital representations from apparatus 10, each of whichmay have different spatial and color characteristics. Color and spatialbasis unification apparatus 14 unifies the spatial and colorcharacteristics of the single-page digital representations and outputsdata for each of the single pages which preferably comprisespixel-interleaved data. Preferably, the single-page data is stored inintermediat storage and is subsequently provided to digital signatureassembly generator 16 as explained below with reference to FIG. 3.

The term “pixel-interleaved data” is defined in Appendix A, attachedhereto.

Color and spatial unification apparatus 14 may comprise aTrans/4,commercially available from Scitex Corporation, Herzlia, Israel, oralternatively may comprise the color and spatial transform apparatusdescribed in Israel Patent Application No. 96957, the disclosure ofwhich is incorporated herein by reference.

The unified page data provided by color/spatial unification apparatus 14is provided to a digital signature assembly generator 16. Digitalsignature assembly generator 16 is operative to provide a digitalrepresentation of the signature by carrying out a full computerized pageimposition function on the unified page data, including provision ofsignature markings such as registration marks, folding marks, cuttingmarks, control strips, as will be described in detail hereinbelow,preferably resulting in a complete digital representation of the fullsignature.

Signature markings are preferably provided as digital files, which aresubstantially analogous to the image files. The signature marking filesare positioned within the signature along with the image files.

Preferably, the digital signature assembly generator 16 includes a“data” double buffer including two buffers, each of which is largeenough to store the digital representation of one single separation of asignature corresponding to the data required for the exposure of an areacovered by one laser beam path. At any given time, one of the twobuffers of the “data” double buffer provides a first line to a digitaloutput device 22 via screen generator 20 and the other one of the twobuffers loads the line next to be provided to the digital output device22 via screen generator 20.

Digital signature assembly generator 16 also preferably provides screencontrol parameters. The screen control parameters may comprise, for eachof a plurality of regions defined within the pages in the signature, aplurality of parameters relating to screen angles corresponding to theplurality of separations. A plurality of screen angles corresponding toa plurality of separations in which a color image is represented istermed herein “a screen angle quartet”. However, it is appreciated thatthe number of separations be used to represent the color image need notbe 4, but rather may also be any other suitable number such as 3.

Preferably, each screen angle quartet is represented by a suitable codeindication, termed herein a “screen control code value”. For example,the code value “1” may represent the screen angle quartet (0, 15, 30,60), the four components representing the screen angle in degrees forthe C, M, Y and K separations respectively. The code value “2” mayrepresent the screen angle quartet (30, 0, 15, 60). A LUT in signatureassembly generator 16 may be used to convert the code values to screenangles according to separation.

Preferably, the digital signature assembly generator 16 includes a“regions” buffer. The screen control data for each signature linetypically comprises a plurality of screen angle quartets correspondingto the plurality of regions and, for each region, the byte count ornumber of pixels over which the region extends in the laser pathdirection. For each path within the signature which overlaps at leastone new region, the screen control code for the new region is convertedto screen angle according to separation and all relevant screen controlparameters are loaded into screen generator 20 with the byte counts ofthe regions and of the spaces between them.

The digital signature assembly generator 16 preferably receives at leastoperator inputs via a signature assembly workstation 18. Signatureassembly workstation 18 may comprise a computer, such as a personalcomputer, running a commercially available impositioning planningsoftware package such as Impostrip, commercially available from UltimateTechnologies Inc., 4980 Buchan St. Suite 403, Montreal, Quebec H4P 158,Canada. Signature assembly workstation 18 is also preferably operativeto perform the other functions described below.

Signature assembly workstation 18 is operative to provide a list offiles to be impositioned, preferably including, for each color imagefile and for each signature marking file, information regarding desiredpositioning thereof on the signature. The information regarding desiredpositioning preferably takes into consideration characteristics of thepost-press equipment such as folding, cutting and binding equipment.Therefore, signature assembly workstation 18 preferably storesinformation regarding the post-press equipment. This file list isprovided to digital signature assembly generator 16.

Preferably, digital signature workstation 18 is operative to receivefrom workstation 10 in at least one of digital pages 12 an operator'sselection of crucial zones, termed herein “areas of interest”, whoseappearances are to be faithfully reproduced. In other words, if thedigital representation within a particular digital page 12 of aparticular area of interest is represented by a particular vector suchas a vector (C, M, Y, K) then it is desired that, when that area isprinted on the press and subsequently analyzed, the resulting digitalrepresentation of the printed area of interest will remain the same (C,M, Y, K).

Preferred methods and apparatus for preserving the appearance of a colorimage are described in Applicant's co-assigned Israel Patent ApplicationNo. 96957.

Signature assembly workstation 18 identifies the information regardingthe areas of interest by signature coordinates and provides theirsignature coordinates to at least press control device 32. Preferably,the above information regarding areas of interest is included in thefile list described above.

The screen control parameters relating to screen angles and the digitalsingle separation signature output of digital signature assemblygenerator 16 are provided to a screen generator 20, which may comprisethe screen generator incorporated in the Raystar or Dolev plotters,commercially available from Scitex Corporation, Herzlia, Israel. Screengenerator 20 is operative to control the writing apparatus of thedigital output device 22. Screen generator 20 preferably comprises a LUTwhich takes into account characteristics of the press 28 such as dotgain. Digital output device 22 may comprise one or more suitablecommercially available digital output devices suitable for producingseparations of entire signatures, such as the ERAY or Raystar plotters,in which the writing apparatus comprises a laser beam. The resultingsignature separations are exposed, thereby to provide a plurality ofplates 24 corresponding to the plurality of separations, which aremounted on the press 28.

The digital signature assembly generator 16 is also operative to providecontrol data to a press set-up device 26 which is operative to set up or“make ready” the press 28 which produces a printed sheet 30. The pressonto which the plate is mounted is typically partitioned into aplurality of strips, also termed ink-key zones, such as 16, . . . , 28or 32 strips. Preferably, the control data provided to the press set-updevice includes ink flow settings for each strip or ink-key zone. Theink flow setting for each strip may be determined according to theaverage dot percentage within that strip, including the dot percentagescorresponding to any signature marks, such as control strips, partiallyor completely overlapping that strip.

According to a preferred embodiment of the invention, a proofing device29 may be provided.

A press control device 32 is preferably provided for inspecting theprinted sheet 30 at at least one location defined by workstation 10 andidentified by signature co-ordinates by workstation 18 in order toobtain at at least that location an indication of the visual appearanceof the image including at least its color content. This information iscompared with the desired visual appearance information as derived fromthe corresponding digital representation 12 of the at least one page atthe corresponding at least one location designated by workstation 10. Onthe basis of this comparison the press control device 32 may modify asnecessary, on-line, at least one press control parameter such as inkflow so as to improve the visual appearance of the printed product 30.

Preferably, the press control device is operative to inspect at leastthe “areas of interest” selected by the operator. The signaturecoordinates of the areas of interest, as well as their visualappearances are provided to press control device 32 by workstation 18,as described above. The visual appearances of the areas of interest,including the color contents thereof, are compared with thecorresponding desired values transferred by workstation 18. The presscontrol device employs the comparison to modify as necessary at leastone control parameter of press 28, such as amount of ink thereby toincrease the correspondence between the parameters of the areas ofinterest as defined by page layout assembly workstation 10 and betweenthe visual appearance of the areas of interest provided by the press 28.

Press control device 32 is also preferably operative to receive anindication of the locations and distances between registration marks inorder to facilitate synchronization of the press 28.

Commercially available press set-up devices 26 include, for example:

CPC3, by Heidelberg Co., Heidelberg, Germany;

Roland RCI and CCI, by Man Roland, Offenbach am Main, Germany; and

PDC Print Density Control, by Komori, Tokyo, Japan.

Press control device 32 may include a plurality of commerciallyavailable press control systems such as the following:

SPM 700, by Gretag Data and Image Systems, CH-Regensdorf, Switzerland;

Calgraph System, commercially available from Celogic, Montpellier,France; and

Image processor model IP-100, by CC1 Inc., Hackettstown, N.J. USA.

The plurality of press control systems typically includes one presscontrol system per separation.

The apparatus of FIG. 1 preferably includes a data base, which mayreside in any suitable location such as the memory of workstation 18,which stores preferred combinations of ink, paper and press parameters.The preferred combinations are preferably combinations which are knownto provide faithful reproduction of color images. The databaseinformation is preferably utilized to modify the operation ofcolor/spatial unification device 14, press set-up device 26.

The printed sheet provided by the press 28 is then provided topost-press equipment such as folding, cutting and binding equipment,using known techniques, thereby to provide a final printed product whichmay comprise a plurality of printed sheets, such as but not limited to abook, newspaper, or magazine.

Reference is now made to FIG. 2, which is a generalized flow chartillustrating an algorithm useful in implementing digital signatureassembly generator 16 of FIG. 1.

As shown in FIG. 2, the first step 50 of the algorithm is to receivefrom signature assembly workstation 18 information regarding theorientation of the image represented by a particular digital file suchas its desired position on the page and whether it is to appear uprightor in a rotated orientation such as upside down. This orientationinformation is preferably included in the file list provided byworkstation 18 as described above. The digital file may comprise anindividual one of digital pages 12 or may alternatively comprise a filerepresenting page markings such as registration marks, folding andcutting marks and control strips. The orientation information ispreferably included in the file list provided to digital signatureassembly generator 16 by signature assembly workstation 18.

A digital representation of a control strip may be generated in the sameway an ordinary image file is generated. Preferred patterns for controlstrips comprise the patterns of commercially available control stripsfrom DuPont (Cromalin process) or from 3M (PrintMatch process).

Conventional patterns for folding, cutting and registration marks areknown and are described in the above-mentioned text, Folding In Practiceby Furler.

In step 51, a plurality of regions is defined which partitions the file.The regions may be of uniform size, such as 50 pixels×50 pixels.Alternatively, the regions may differ in size, the size of each regionbeing a function of the amount of variation of the color values of thepixels within that region, such that, within each region, there is avery small amount of variation between the pixel values.

Step 52 is to determine, from operator input received via signatureassembly workstation 18, whether the page or file requires rotation by180 degrees. The page or file is then stored in blocks, using thenon-rotating block providing procedure of FIG. 3, if 180-degree rotationis not required, or using the rotating block providing procedure of FIG.4 if 180-degree rotation is required. For as long as files remain to beprocessed (step 54), the above process, comprising input step 50,partitioning step 51, decision step 52 and the go-to steps, termedherein “the first pass”, is repeated. Step 54 may be implemented byreferring to the list of files to be impositioned on the signature whichmay be provided by a user via signature assembly workstation 18 asexplained above.

At the end of the first pass, a second process, termed herein “thesecond pass” and described herein with reference to FIG. 5, is initiatedfor each separation.

Reference is now made to FIG. 3 which is a flow chart of a non-rotatingblock providing procedure useful in conjunction with the algorithm ofFIG. 2. As shown, the procedure of FIG. 3 comprises a first step 60 inwhich a block index i is initialized and assigned an initial value of 1,corresponding to the first block of the page or file being processed.

In step 62, block i is brought from an individual file from among thelist of files to be impositioned. The individual file is provided to thedigital signature assembly generator 16 by color/spatial unificationunit 14, as shown in FIG. 1. Block i is stored in a local memory devicesuch as a solid state memory. The block i information stored in thelocal memory device preferably includes the pixel interleaved datadefining the corresponding portion of the color image.

In step 66, a screen subroutine is performed in which a screen anglequartet, or a screen control code value representative thereof, asexplained above, is computed for each region. The screen subroutine isdescribed in detail hereinbelow with reference to FIG. 6.

Step 68 is a decision step in which the algorithm branches depending onwhether the i-th block is the last block or not. If it is not, then if 4blocks have accumulated in local memory (step 70), the 4 blocks, each ofwhich contain information regarding all four separations of a portion Pof the page, are reorganized (step 72). Each of the reorganized blockscontains information regarding a single separation of a portion of thepage four times as large as P. Step 72 is not performed until 4 blockshave accumulated in local memory. The reorganized blocks are stored in abulk memory unit such as a disk. The block index i is incremented (step74) and the algorithm is repeated.

If the block index i in step 68 corresponds to the index of the lastblock of the page or other file, the blocks in local memory, which maynumber 1, 2, 3 or 4 blocks, the last of which may not be full, arereorganized (step 76). The blocks in local memory are reorganized into 4complete or partial blocks. The reorganized blocks or partial blocks arestored in a bulk memory unit such as a disk. Step 76 is thereforesimilar to step 72, except that the 4 “separation” blocks are notnecessarily complete blocks.

Step 76 is followed by step 78, in which screen-angle quartets computedfor particular regions in step 66 are transferred from “region” localmemory to bulk memory.

Reference is now made to FIG. 4 which is a flow chart of a rotatingblock providing procedure useful in conjunction with the algorithm ofFIG. 2. The procedure of FIG. 4 is similar to the procedure of FIG. 3,analogous steps therefore having been given identical referencenumerals, with the following differences. In FIG. 4, initialization step60 is replaced by an initialization step 80 in which block index ireceives an initial value indicative of the last block of the page orfile.

Step 80 is followed by step 82 in which block i is brought in and isstored in local memory in reverse. In other words, in the stored blocki, the row order and the pixel order within each row of the block areinverted, relative to the original block i, since the page or file is toappear “upside-down” on the plate.

Step 84, in which screen angle quartets are computed for the variousregions, is similar to step 66 and is described in detail below withreference to FIG. 6.

Decision step 68 of FIG. 3 is replaced by decision step 86 whichdetermines whether i points to the first block. Index incrementationstep 74 of FIG. 3 is replaced by index updating step 88 in which 1 issubtracted from i.

As noted hereinabove, when the “first pass” procedure of FIG. 2 iscompleted, a second process, termed herein “the second pass”, isinitiated for each separation, which provides digital informationregarding the entire separation of the signature to screen generator 20of FIG. 1. Preferably, the first and second passes are pipelined.

Reference is now made to FIG. 5 which is a flowchart of a preferredembodiment of the “second pass” procedure. As shown in FIG. 5, the“second pass” procedures begins with loading step 98 in which the screenangle quartets computed in step 66 of FIG. 3 or step 84 of FIG. 4 areloaded to local memory.

A laser beam path index initialization step 100 follows, in which anindex k of the laser beam path of the digital output device 22 isinitialized to correspond to the first laser beam path of digital outputdevice 22. In step 102, reference is made to the list of files to beimpositioned, which list includes information regarding desired marginsand the desired arrangement of the pages on the signature, in order todetermine whether laser beam path k will encounter any files or pages.If not, step 104 is operative to clear the entire buffer of the doublebuffer which stores path k, as by filling with zero's. If there arefiles along laser beam path k, a current file index or pointer isdefined to refer to the first file on path k (step 108). In step 110,the offset of the laser beam path k of digital output device 22 of FIG.1 is set with the corresponding coordinate of the first file on path k.Also, the stored information corresponding to the blank or offsetportion of path k is cleared, in order to overwrite obsolete informationfrom previous paths. As explained above, the information regarding pathk is stored in one of the two buffers of the double buffer withindigital signature assembly generator 16.

Decision step 112 determines whether the current file data for path k isstored in the local memory. If not, the next block of the current fileis brought to the local memory from the bulk memory (step 114). The pathk data is taken from that block and is stored in an individual one ofthe buffers of the double buffer within digital signature assemblygenerator 16 (step 116).

Decision step 118 determines whether the current file is the last filealong path k. If not, the gap between the current file and the next filealong path k, as indicated by the file list, is cleared (step 120). Thecurrent file index or pointer is updated to refer to the next file onpath k and the algorithm returns to step 112.

If the result of decision step 118 is that the current file is the lastfile along path k, a subroutine is employed (step 124) which isoperative to output path k−1 (for k>=2). If k=1, steps 124 to 128 arenot performed. A preferred embodiment of the subroutine of step 124 isdescribed below with reference to FIG. 7. The end of path k is marked instep 126. If k is not the last laser beam path (decision step 128), thelaser path index k is incremented and the algorithm returns to step 102.If k is the last laser beam path, the subroutine of FIG. 7 is performedon path k, thereby to provide an output indication of path k.

The blocks of FIG. 5 define a data organization subalgorithm 140 and anoutput providing subalgorithm 142. Preferably, the output providingsubalgorithm 142 is pipelined with the data organization subalgorithm140 to allow continuous operation of the digital output device 22. Inother words, while the data of path k is being organized, an outputindication of path k−1 is being provided.

Reference is now made to FIG. 6 which illustrates a preferred embodimentof the screen subroutine performed at step 66 of the algorithm of FIG.3. As explained above, the screen subroutine of FIG. 6 is operative toexamine each pixel of block i and compute a screen angle quartet,corresponding to the plurality of separations, for each region whoselast pixel falls within block i.

Generally speaking, the screen angle quartet for each region may becomputed as follows: For each region, a plurality of counters ismaintained corresponding to the plurality of code values defined by thescreen code. In other words, each time a pixel within that region isfound to have been assigned a particular screen code value, the countercorresponding to that screen code value is incremented. When all thepixels within the region have been examined, the most frequentlyoccurring screen code value is assigned to the region as a whole and istermed herein the “screen control code”. It is appreciated that anysuitable function may replace the above mode function in computing thescreen control code value of a region as a function of the screen codevalues of pixels within the region.

The steps of the subroutine may be as follows:

In step 150, a screen angle quartet, or a screen code corresponding to aparticular screen angle quartet, is assigned to a pixel. According to apreferred embodiment of the present invention, the screen angle quartetof each pixel is a function of the lowest density component of the pixelvalue. A sample function for CMYK pixels is as follows:

Lowest density component Screen angle quartet (in degrees) of pixelvalue C M Y K C 15 0 30 60 M 0 15 30 60 Y 30 0 15 60 K 60 0 30 15

In step 151, a pixel is stored in the “data” local memory.

In step 152, the region in which the pixel is included is identified. Instep 154, the counter corresponding to the particular code value of thepixel, on the one hand, and corresponding to the particular region, onthe other hand, is incremented.

If the pixel processed above is the last pixel of a region (step 156),then a screen control code value corresponding to a screen angle quartetmay be assigned to the region (step 158). As explained above, the screencontrol code value of the region is preferably the screen code valuewhich occurs most in that region. The region screen code value may bestored in “region” local memory.

If the pixel processed above is not the last pixel of the block (step160), the algorithm, starting from step 150, is repeated for the nextpixel of the block.

Step 84 of FIG. 4, in which a screen angle quartet is computed for thevarious regions, is now described in detail. The subroutine of step 84may be similar to the subroutine of FIG. 6, as described above. However,in step 151, pixels are stored in the “data” local memory in reverseorder. Also, in step 158, region screen control code values are storedin the “region” local memory in reverse order.

Reference is now made to FIG. 7, which is a preferred embodiment of thepath outputting subroutine of steps 124 and 130 of the algorithm of FIG.5. The path outputting subroutine preferably comprises the followingsteps:

In step 172, screen control code values are fetched from local memoryfor all new regions intersecting the current path of the laser beam ofthe digital output device 22 of FIG. 1. The term “new regions” hererefers to regions which intersect the current path of the laser beam butdo not intersect the previous path of the laser beam.

In step 174, each screen control code value is converted into the screenangle for the current separation or component of the screen anglequartet.

In step 176 the screen parameters are loaded into screen generator 20for each region together with the byte counts corresponding to theregion heights and the gap sizes in between regions. An outputindication of the current path of the current separation is provided instep 178.

Step 180 is a decision step which determines whether the current pathdoes or does not intersect any regions which did not intersect theprevious path. If not, in other words, if the regions intersecting thecurrent path are the same as the regions intersecting the previous path,steps 172, 174 and 176 may be bypassed because the screen informationloaded in screen generator 20 is still correct.

In the present specification, the term “page” is intended to include anyunit included within a signature which may include representations of anactual page such as a page of a book as well as representations ofsignature markings and control strips. The term “plate” is intended torefer to any unit of production of a printing device such as a pressincluding, but not limited to, a print forme as defined on page 33 ofthe above mentioned text “Folding in Practice” by A. Furler.

It will be appreciated by persons skilled in the art that the presentinvention is not limited by what has been particularly shown anddescribed hereinabove. Rather the scope of the invention is defined onlyby the claims which follow:

1. A closed loop control system for printing an image on a sheet using acolor printing press comprising: a) means for receiving signaturecoordinates of said image; b) means for receiving desired visualappearance related to said signature coordinates; wherein said means forreceiving signature coordinates and said means for receiving desiredvisual appearance serve as input for said control system; c) means forextracting information from said printed sheet for use as feedback forsaid control system; and d) means for changing press parametersaccording to said input and said feedback to improve the visualappearance of subsequent prints of said image.
 2. The system of claim 1wherein said means for receiving signature coordinates comprises meansfor receiving page coordinates and means for translating said pagecoordinates to signature coordinates.
 3. The system of claim 1 whereinsaid means for receiving signature coordinates and said means forreceiving desired visual appearance comprise operator input.
 4. Thesystem of claim 1 wherein said visual appearance includes at leastcolor.
 5. The system of claim 1 wherein said press parameters include atleast ink flow.
 6. The system of claim 1 wherein said control systemcomprises a plurality of control systems, each for controlling theprinting operation of one color separation of said image.
 7. A method ofcontrolling, in a closed loop, the printing of an image on a sheet usinga color printing press, comprising the steps of: a) receiving at leastone input signal including signature coordinates defining at least onearea of interest in said image and desired appearance information linkedto said at least one area of interest; b) receiving at least onefeedback signal including information extracted from said printed sheet;c) comparing said feedback signals and said input signals; and d) usingthe result of said comparison to modify at least one press controlparameter, to improve the visual appearance of subsequent prints of saidimage.
 8. The method of claim 7 wherein said at least one press controlparameter includes ink flow.
 9. The method of claim 7 wherein saidvisual appearance includes at least color.
 10. The method of claim 7wherein said color printing press comprises a plurality of printingstations and wherein said control operation is applied to each of saidprinting stations for printing a plurality of color separations.
 11. Aclosed loop control system for printing an image on a sheet using acolor printing press, comprising: a) at least two printing stations,wherein each printing station prints one color separation; b) means forreceiving signature coordinates of said at least two color separations,wherein said means for receiving signature coordinates serve as inputfor said control system; c) means for extracting information from saidprinted sheet for use as feedback for said control system; and d) meansfor changing press parameters according to said input and said feedbackto improve the visual appearance of subsequent prints of said image. 12.The system of claim 11, wherein said means for receiving signaturecoordinates comprises means for receiving page coordinates and means fortranslating said page coordinates into signature coordinates.
 13. Thesystem of claim 11, wherein said means for receiving signaturecoordinates comprises means for receiving coordinates of at least oneregistration mark.
 14. The system of claim 11, wherein said pressparameters include press registration parameters.
 15. A method ofcontrolling, in a closed loop, the printing of an image on a sheet usinga color printing press, comprising the steps of: a) receiving at leastone input signal including signature coordinates defining at least oneregistration mark of one color separation of said image; b) receiving atleast one feedback signal including registration information extractedfrom said printed sheet; c) comparing said feedback signal and saidinput signal; and d) using the result of said comparison to modify atleast one press registration parameter, to improve the visual appearanceof subsequent prints of said image.
 16. The method of claim 15, whereinsaid color printing press comprises a plurality of printing stations andwherein said modifying affects at least one of said plurality ofprinting stations.
 17. The system of claim 2 wherein said means forreceiving signature coordinates and said means for receiving desiredvisual appearance comprise operator input.